The present invention is directed to the art of glass manufacturing and more particularly is directed to an improvement in the art of glass manufacturing wherein hot iron or hot metal molds are utilized.
Glass articles are manufactured in the glass industry by one of two non-analogous techniques. That is, the formation can be brought about by the utilization of paste mold technology or it may be formed by the non-analogous method of employing hot iron or hot metal mold technology, the latter hereinafter being referred to as hot metal mold technology. With regard to this distinction between paste mold technology and hot metal mold technology, and also for a description of the techniques for forming glass articles reference may be had to Glass Engineering Handbook by E. B. Shand, McGraw Hill Book Company, 1958, and the Handbook of Glass Manufacture, Volume 1, Ogden Publishing Company, 1953. Paste mold technology involves a rotational speed differential between the paste mold and the glass, that is, there is relative rotation of the paste mold and the glass with the paste mold being run in a wet condition by the use of water. The paste itself is generally some form of adherent carbon which is porous and thereby capable of absorbing the water. It is generally indicated that, with regard to the paste mold operation, the glass is formed against a boundry layer of steam which functions as a cushion. One of the characteristics of the glassware produced by using paste mold technology is that because of the relative rotation of the paste mold and the glass, the final glass articles have no mold seam. U.S. Pat. No. 2,573,337 discloses the use of a cured organopolysiloxane for use on a paste mold when forming, for example, glass bulbs on a bulb or ribbon machine.
In the non-analogous technique of forming glassware by the use of hot metal molds, molten formable glass is first formed into a parison, or blank, by being brought into contact with a glass-contacting, or glass-forming, cavity-defining surface of a parison, or blank, mold and this parison, or blank, is then subsequently formed into the final article in a blow mold by contact with a glass-forming, or glass-contacting, cavity-defining surface thereof. The formation of the parison, or blank, and the formation of the final article from the parison or blank is accomplished without relative rotation of the glass and the respective molds. Typical of the glass forming apparatus, which operate with hot metal molds are the conventional I.S. glass forming machines, which may operate either on a blow and blow type of operation or a press and blow type of operation, and the Owens glass forming machine. Inasmuch as the use of hot metal mold forming does not employ a steam cushion, it is generally considered that in such forming techniques the glass is in contact with the glass-forming, cavity-defining surface of the mold. Those skilled in the art know the importance of the characteristics, e.g. the cavity-defining surface characteristics, of a hot metal mold to the proper operation of forming process. The interior surface, that is, the glass-contacting or glass-forming, cavity-defining surface thereof, must possess characteristics, including proper heat transfer and proper release of the glass, so as to avoid undesirable quality defects. The hot metal molds must also run hot enough to avoid sudden cooling of the glass which would otherwise result in the formation of checks. Balanced against this, it will be appreciated that the hot metal molds must not run too hot because the glass will tend to stick to the cavity-defining surface of the mold and produce a final article having a quality defect characteristic of such sticking. In an attempt to help the overall operation, of forming glass using hot metal molds, it is commercial practice to dope, or swab, these molds to assist glass release with such materials as oil, graphite, greases, sulphur, rubber, old shoe heel and the like. This type of swabbing, or doping, has its readily apparent deficiencies and limitations. For example, such doping or swabbing is generally done based on the subjective evaluation of independent glass machine operators and therefore, is not conductive to reliability. Another deficiency of this technique is that, typically, these materials are carried in volatile organic carriers which carriers, upon contact with the hot metal mold, flash off and contaminate the general forming area with pollutants.
Czechoslovakian Pat. No. 128,236 entitled "Lubricants for Glass Molds", and the corresponding abstract thereof appearing in Chemical Abstracts, Volume 70, 1969, page 195, Abstract No. 108868Y, disclose a glass mold lubricant formed from an organopolysiloxane and colloidal graphite. Attention is also invited to Chemical Abstracts, Volume 60, Abstract No. 763D, entitled "Coatings Which Prevent Sticking of Molten Glass to a Mold" which discloses colloidal graphite and a silicone. Colloidal graphite is a permanent suspension of finely ground natural, or manufactured graphite, dispersed in a liquid carrier and is usually marketed as a dispersion or suspension concentrate of about 10 to 20% by weight graphite. The particle size of this colloidal graphite is on the order of 1 micron and finer and hence it will be appreciated that this is an extremely small, high surface area material. As will be seen in the examples which follow, the use of colloidal graphite is not satisfactory inasmuch as the wear rates obtained using such a material are not compatible with the need in the glass industry, in employing hot metal mold technology, for a high speed, high quality, low cost glass forming operation. With regard to a description of colloidal graphite, reference may be had to Kirk-Othmer, Encyclopedia of Chemical Technology, Volume 4, 1964, at page 325.
In U.S. Pat. No. 3,347,650, some of the deficiencies of swabbing molds are pointed out. This patent then provides for a solid film lubricant on a mold which, for example, is formed of lead monoxide powder in admixture with graphite. The undesirability of such an approach because of the toxicity of lead will be readily apparent to those skilled in the art.
German Offenlegungsschrift No. 2,303,861 is directed to a sprayable lubricant for glass container molds to be sprayed after every molding cycle. The sprayable lubricant describes the use of graphite in a finely divided solid organic polymer with a ratio of graphite to organic polymer being about 1:0.1 to 1:15. Offenlegungsschrift No. 2,303,861 has no recognition whatsoever, however, of forming a permanent or solid film lubricant on the mold as a glass-contacting or glass-forming, cavity-defining surface thereof since it is directed to spraying after every molding cycle.
From the foregoing, it will be appreciated that there is a need in the art of forming glass, employing hot metal mold technology, wherein in the forming steps there is no relative rotation of the glass and the mold, for providing a glass release agent or lubricant on the cavity-defining surface of the mold which does not present toxicity problems, and which results in high production efficiencies with the formation of high quality wear at low costs, and which further does not require emission into the adjacent environment of the significant amounts of pollutants presently emitted by swabbing with the dopants generally commercially employed.
In accordance with this invention, there is provided an improvement in methods for forming glass articles wherein the glass is formed by a hot metal mold operation in which the cavity-defining surface of a blank mold is provided with a layer of a glass-contacting, cavity-defining solid film lubricant or solid film glass release agent. This layer satisfies the needs in the art in that it has the needed operational characteristics so as to provide improved efficiencies in glass formation at higher quality and at lower costs and which greatly minimizes, if not eliminating, the environmental pollution in the forming areas which previously resulted from the necessity of swabbing.
Thus there is provided an improvement in processes for forming glass articles of the type wherein formable hot glass is formed into a blank, or parison, by contact with a glass-forming, cavity-defining surface of a hot metal blank, or parison mold, and the blank is then converted into the final glass article, such as, for example, a glass container, by contact with a glass-forming, cavity-defining surface of a blow mold; the improvement essentially resides in employing, as the glass-forming, or glass-contacting, cavity-defining surface of the hot metal blank mold, a layer or film of a solid lubricant, or glass release agent in which the layer is essentially non-colloidal graphite dispersed in a cured thermoset organopolysiloxane binder, with the graphite being present in an amount sufficient to provide a glass releasing quality to the layer. That is, instead of forming the parison, or blank, in a parison, or blank, mold by contact with the cavity-defining, metal surface of that mold, this cavity-defining surface is now provided with and carries a solid film lubricant layer. This layer is formed by applying a dispersion of the non-colloidal graphite in an organic solvent solution of a solvent-soluble, further-curable, thermosettable organopolysiloxane unto the cavity-defining metal surface of the mold to provide a cavity-defining layer whose expand surface will be used for contact with glass in the glass forming step. The non-colloidal graphite contemplated herein has a weight percent size distribution in which substantially all, for example, 95% by weight of the particles are in excess of 1 micron in contrast to colloidal graphite, in which substantially all of the particles, or at least a major amount such as, for example, 50-70% or so, of the particles by weight are less than 1 micron.
In another embodiment of this invention in addition to the blank mold having the solid layer indicated above, the blow mold is likewise provided with a solid film lubricant layer in the manner indicated above, with the size of the graphite in the layer on the blow mold being no larger than the size of the graphite employed for forming the blank mold layer and, preferably, in order to provide an extremely smooth surface on the glass article, the size of the graphite will be smaller than that employed in forming the layer on the blank mold.
As generally indicated above, the solid film glass lubricant layer is formed on the blank mold, or the blow mold, by applying onto the contour of the normal glass-contacting, cavity-defining metal surface of a conventional blank mold, or blow mold, a dispersion of the graphite in an organic solvent solution of a thermosettable, further-curable, organopolysiloxane. Conventional techniques will be advantageously employed in preparing the cavity-defining metal surface of the blow mold or blank mold to receive the dispersion. That is, in the preferred embodiment the mold surface will be prepared using conventional techniques of grit-blasting and priming. In the priming operation, conventional primers may be employed, such as, for example, the phosphates, with particularly suitable conventional primers being amino organosilicon compounds. Representative of satisfactory primers are those set forth, for example, in U.S. Pat. No. 3,088,847, as well as in British patent specification No. 952,992. Particularly, suitable primers are the amino organosilicon compounds exemplified in British Pat. No. 952,992, at Table I, such as, for example, the amino silicon material designated as D as well as combinations of these amino organosilicon compounds with the epoxy compounds therein indicated. The application of the graphite dispersion in the organic solvent solution of the further-curable, thermosettable, hardenable organopolysiloxane may likewise be done using conventional techniques, such as flow-coating and spray-coating. In passing, it should be mentioned that when reference is made herein to the graphite particle size, the size referred to is the size of the graphite added to the organopolysiloxane solution to form the dispersion. After applying the dispersion to the cavity-defining metal surface of the blow mold and after evaporation of the solvent, final curing of the organopolysiloxane is effected so as to form the solid film lubricant layer of graphite dispersed, or bound, in the thermoset, cured, hardened, organopolysiloxane binder and having a glass-contacting, cavity-defining surface. Generally, the thickness of the layer of the solid film lubricant once formed on the blank mold, which layer will have the general contour and configuration of the previous cavity-defining metal surface, will be balanced so as not to be so thick as to cause a flaking off of the film during utilization and glass formation, and yet not to be so thin that the film may result in premature failure due to film rupture. In general, it will be found that thicknesses of the final layer on the order of about 2 to about 3 and 1/2 mils and, most desirably, a thickness in the range of about 2 to 3 mils, for example, about 2 and 1/2 to 3 mils will provide excellent results for the blank mold or parison mold; with the blow mold the desired thickness will be about 1 to 2 mils, preferably about 1 to about 1.5 mils. For some of the general considerations involved in forming and applying solid lubricants reference may be had to Solid Lubricants by M. E. Campbell, J. B. Loser and E. Sneegas, NASA, Washington, D.C. May 1966, pages 7-17.
The organopolysiloxanes employed are the solvent-soluble, further-curable, hardenable, thermosettable organopolysiloxanes. These materials are well known in the art and are hydrolysis and condensation products of hydrolyzable silanes. That is, they are hydrolysis and condensation products of silanes having hydrolyzable groups thereon such as for example halide groups, typically the chloride group, or alkoxide groups, in which the alkyl portion of the alkoxide has from 1 to about 5 carbon atoms. The preferred thermosettable, solvent-soluble, further-curable organopolysiloxanes will be organopolysiloxanes in which the organo groups are lower alkyl groups, for example, C.sub.1 to C.sub.3 alkyl groups or phenyl groups. These materials may be prepared by hydrolysis and condensation of the respective hydrolyzable silane alone, or they can be hydrolysis condensation products of mixtures of the hydrolyzable silanes, or the organopolysiloxanes can simply be admixtures of 2 or more further-curable, thermosettable organopolysiloxanes. The preferred organopolysiloxanes are the thermosettable, hardenable, further-curable, methylphenylsiloxanes. As is well known in the art, these types of further-curable, thermosettable organopolysiloxanes can be described by reference to their R:Si ratio, wherein R designates the moles of organic radicals directly attached to the silicon atoms. As contemplated herein in the preferred practice of this invention, the organopolysiloxanes will have an R:Si ratio of 1:1, to less than about 2:1. At ratios higher than 2:1 the organopolysiloxanes are typically oils and are not the further-curable, thermosettable type of organopolysiloxane which will be advantageously employed herein. Highly desirable results will be obtained by using a further-curable, thermosettable organopolysiloxane having an R:Si ratio of about 1:1, or more, up to about 1.6:1 with especially fine results being obtained using an R:Si ratio between about 1.2:1 to about 1.6:1. As previously indicated in the preferred embodiment, the further-curable, thermosettable, organopolysiloxane is a methylphenylsiloxane. Thus, in the preferred practice, as will be readily apparent from the R:Si ratios given above, these siloxanes can be prepared by hydrolyzing the appropriate hydrolyzable silanes to obtain the desired R:Si ratio. For example, these materials can be obtained by the hydrolysis and condensation of a condensable and hydrolyzable monomer mixture of methyltrichlorosilane, phenyltrichlorosilane, and dimethyldichlorosilane. Alternatively, they can be prepared by the hydrolysis and condensation of methyltriethoxysilane, phenyltriethoxysilane and dimethyldiethoxysilane. Of course, as indicated above, other alkoxy silanes and other halosilanes may be employed as well as can mixtures thereof. Additionally, as indicated above, the R:Si ratio may be 1:1 which indicates that the organopolysiloxane can be a lower alkyl polysiloxanes, such as, for example, methylpolysiloxane manufactured from, for example, methyltrichlorosilane or methyltrialkoxysilanes, such as, for example, methyltriethoxysilane or the organopolysiloxane can be a phenyl organopolysiloxane such as that manufactured from, for example, a phenyltrichlorosilane or a phenyltrialkoxysilane, such as, for example, phenyltriethoxysilane or admixtures thereof.
The organic solvent for the further curable, thermosettable, hardenable, organopolysiloxane which is used to form the organic solvent solution thereof, and into which there is then dispersed the graphite as herein contemplated will be routinely selected by those skilled in the art. Exemplary solvents include ethyl alcohol, propyl alcohol, benzene, ethers, ketones, for example, acetone, mixtures thereof, mixtures for example of mineral spirits, isobutylacetate and ethylene glycol monomethyl ether, and aromatic solvents such as, for example, xylene and toluene. Particularly, fine results will be obtained, for example, when using xylene. The concentration of the further-curable, thermosettable organopolysiloxane in the organic solvent solution, will likewise be routinely selected by those skilled in the art but quite convenient operation will be obtained by employing, for example, an organic solvent solution of about 10 to about 35% by weight of organopolysiloxane solids, with quite convenient operation being obtained using a solution on the order of about 25 to 35% by weight of organopolysiloxanes. For example, an especially preferred system is around 30% by weight of organopolysiloxane in xylene.
Dry particulate, non-colloidal graphite is intimately combined with the solvent solution to form a dispersion of the graphite therein. Advantageously, the weight ratio of the graphite to the organopolysiloxane solids in forming the dispersion will be on the order of about 0.8:1 to about 2:1, preferably about 1:1 to about 2:1 with quite outstanding results being obtained by using a ratio of about 1:1 to about 1.75:1 and a ratio of 1.5:1 being superior. With regard to the size of the graphite, reference is made to the attached graph which is a conventional semi-log size plot for various graphites of the weight percent (ordinate axis) of particles which are coarser than a prescribed particle diameter (abscissa axis) in microns. For convenience the graph includes, as dotted lines, the 90% coarser and 10% coarser lines of the ordinate axis extending respectively from curves B to F. The curves are based on a Coulter Counter size analysis and when reference is made herein to size, there is meant a Coulter Counter size analysis. The graphite, which is surprisingly and most advantageously employed in forming the glass-contacting or glass-forming, cavity-defining surface of the solid film lubricant layer, or coating, on the blank mold will be graphite having a size distribution curve in which the curve between the 90% coarser and 10% coarser portion falls in the area approximately defined by: (1) curve B, (2) curve F, and (3) the 90% and (4) the 10% coarser lines of the ordinate axis and, preferably in the area approximately defined by: (1) curve C, (2) curve E, and (3) the 90% and (4) the 10% coarser lines of the ordinate axis. Quite outstanding results are obtained by using graphite having a size distribution curve within the area approximately defined by curve B and curve F and, preferably, within the area approximately defined by curve C and curve E, with graphite having a size distribution on the order of curve D being especially preferred. The foregoing generally describes the graphite to be employed on the blank mold solid film lubricant layer. In the embodiment wherein the blow mold is likewise provided with a layer of the solid film lubricant of graphite decreased in the thermoset, cured organopolysiloxane, the graphite employed in forming the blow mold layer will be of a size no larger than that employed in forming the layer on the blank mold and, preferably, will be smaller in size; if, for example, the graphite employed for the blow mold is of a size larger than about curve D it will be desirable to smooth out the surface of the solid film lubricant layer by rubbing with emery paper prior to use.
The dispersion of the graphite in the organic-solvent solution of the further-curable, thermosettable, organopolysiloxane which is applied onto the cavity-defining metal surface of the blank mold, or blow mold, may also include other materials. These other materials include, for example, materials which promote the curing rate of the organopolysiloxane in which case the cure promoters will be present in an amount sufficient to promote such curing. Typically, the cure promoters will be present in an amount less than about 15% by weight based on organopolysiloxane solids. The cure promoters which are employed will be routinely selected by those skilled in the art and are materials which are conventionally employed for curing further-curable organopolysiloxanes. Particularly suitable cure promoters which are per se known for curing organopolysiloxanes are the melamine formaldehyde, partial condensate resin, which term comprehends within its scope the alkylated melamine formaldehyde partial condensate resins. These alkylated melamine formaldehyde resins are melamine formaldehyde types in which alkylation is effected with lower alkyl alcohols, or mixtures thereof, such as, for example, the C.sub.1 to C.sub.5 alkyl alcohols. One suit suitable material is that supplied by the Koppers Chemical Company as their Koprez 70-10 butylated melamine formaldehyde partial condensate resin. When using, for example, the melamine formaldehyde partial condensation resins to promote the curing of the organopolysiloxane, quite satisfactory results will be obtained by using between about 0.5% or 1% by weight up to about 14 or 15% by weight, of the melamine formaldehyde partial condensate resin based on organopolysiloxane solids by weight. Excellent results will be obtained with no adverse effect on the wear rates or operation of the glass forming process by, for example, employing about 13 to 14% by weight of the melamine formaldehyde partial condensate resin, based on organopolysiloxane solids. Other particularly suitable cure promoters include the phosphonic acids such as those set forth in U.S. Pat. No. 3,654,058; with phenyl phosphonic acid being especially preferred, for example, in an amount of about 5% by weight based on organopolysiloxane. Additionally, the dispersion may include conventional adjuvants such as, for example, conventional thixotropes which are employed to adjust the rheology of the dispersion to give the flow best suited to the manner in which the dispersion is applied onto the cavity-defining metal surface of the mold. Typically, these thixotropes are present in rather small amounts, such as, for example, less than 2 or 3% by weight, based on organopolysiloxane solids. These thixotropes are well known in the art with one suitable material being Thixin R, supplied commercially by the Baker Castor Oil Company, which is a hydrogenated castor oil. Other suitable thixotropes which may be employed include, for example, those commercially available from Kelco Company under their Soloid designation.
After the dispersion of the graphite in the organopolysiloxane solution is applied onto the cavity-defining surface of a conventional blow mold, or blank mold, the solvent is allowed to evaporate and then the organopolysiloxane is heated for a sufficient time and at a sufficient temperature to convert it to a hard, cured, thermoset organopolysiloxane. This results in the formation of the layer, or coating, which has a glass-contacting, or glass-forming, cavity-defining surface which functions as the solid film lubricant herein and which layer includes the graphite dispersed in the cured organopolysiloxane binder. Of course, as will be appreciated, if a cure promoter is employed and/or a thixotrope the binder will likewise include these materials. Generally, as will be seen herein, the binder will be on the order of at least about 80% by weight, and typically at least about 85% by weight of the thermoset, cured, organopolysiloxane.
While the present invention has been described with sufficient particularity above to enable those skilled in the art to routinely make and use the present invention there nonetheless follows several examples which will further exemplify the invention with regard to blank molds.
For purposes of convenience in the following examples when reference is made to R-630 resin this refers to a hardenable thermosettable organopolysiloxane resin solution (60% by weight resin solids in xylene) in which the organic groups are methyl and phenyl groups, i.e., a methylphenylsiloxane, and wherein the ratio of these organic groups i.e. the ratio of methyl and phenyl radicals per silicon atom (R:Si ratio) is about 1.4 and wherein the ratio of methyl to phenyl radicals on a mole basis is about 3.3:1, both of these values being based on analysis. Additionally, for purposes of convenience various graphites are referred to in the examples. The graphite designated 007-S corresponds to the size analysis generally exemplified by curve C, the graphite designated A-98 generally corresponds to the size analysis designated by curve D and the graphite designated UC-38 corresponds to the size analysis designated by curve E. These curves represent the averages of at least two separate Coulter Counter size analysis for each graphite.
The specific Coulter Counter employed was a Model T type manufactured and supplied by Coulter Electronics Inc.; the technique employed was the conventional technique employing multiple apertures of 400, 140, 50, and 30 microns and employed an electrolyte of about 4 grams of lithium chloride in 100 ml of a solution of the lithium chloride in methanol. The 007-S and A-98 graphite was commercially obtained from Asbury Graphite Mills Inc., and the No. 38 material was obtained from Union Carbide Chemical Company. In general, these graphites may be described as electric furnaces, or synthetic, graphite and are supplied as dry particulate material. Suitably, for convenience in the following examples reference is made to a cure promoter, or catalyst, designated Koprez 70-10. This resin is commercially available from Koppers Chemical Company and is an organic solvent solution of a butylated melamine formaldehyde partial condensate resin, the resin solids being about 80% by weight in n-butyl alcohol.
In the following examples, the blank molds were prepared by employing a vertically reciprocating spray. That is, the split molds were closed and the cavity-defining surface thereof was sprayed by employing a vertically reciprocating 360.degree. spray nozzle. Specifically, a carriage was provided which was pneumatically moved in a vertical direction and this carriage carried a vertically disposed tubular member to which was attached, at its lower portion, a 360.degree. spray nozzle; the cavity portions of the mold were sprayed by supplying the anhydrous dispersions, as described in the examples, to the spray nozzle through the tubular member with spraying transpiring while the carriage was pneumatically moved upwardly. Prior to spraying, the cavity-defining surfaces of the mold they were first conventionally grit-blasted and then primed with conventional primers. The specific material employed is that commercially supplied by Union Carbide Corporation, as their material A.P. 132. The molds, after curing the organopolysiloxane so as to provide a layer on the mold, of graphite dispersed in a cured organopolysiloxane binder, having a glass-forming, or glass-contacting, cavity-defining surface, were employed as the blank molds in a pilot plant facility having conventional I.S. machines to form what is referred to in the trade as GB-121 glass bottles. The I.S. machines used the prepared hot metal molds and were operated in a blow and blow mode of operation, although it will be apparent that a press and blow mode of operation is equally satisfactory. That is, in the forming operation a formable hot glass charge was formed into a parison, or blank, in a parison or blank mold with no relative rotation thereof and then the parison, or blank, was formed into the final bottle in a blow mold, again with no relative rotation of the glass and blow mold. In the examples which follow the neck rings and bottom plates were not provided with the solid film lubricant layer contemplated herein although it will be apparent they can be.
Finally, in the examples which follow, wear rates are given, as well as are mold lifetimes. Wear rate is the parameter used to evaluate the quality of the blank mold solid film lubricant layer. The wear rate is expressed as a loss of coating thickness per unit time and is reported in mils/hour. Thus, the larger the number the greater the amount of wear. The blank molds are coated and the thickness of the coating, or solid film lubricant layer, is measured at six points, three on each side of the split cavity mold. The points used are one inch from the top of the cavity, one inch from the bottom, and the approximate middle of the mold blank. The same points will be used when thickness measurements are made again after wearing has occurred by use in the hot metal mold, glass-forming process. The coating thickness is measured by using a magnetic thickness gauge type 7000 manufactured by H. Tinsley and Company Ltd., London, England. The molds are then mounted in the glass forming machine, here an I.S. bottle forming machine, and bottles are made until they are no longer of the desired quality, i.e. until the mold coating fails. The failure of the mold coating is that point at which any bare metal of the mold becomes exposed. This exposure typically causes sticking of the glass to the exposed metal surface of the mold creating a defect in the glass surface. The blank mold is then removed from the machine and coating thickness measurements are made at the same six points. The decrease in coating thickness at each location divided by the time the mold was in use is the wear rate at that location. The average of the six wear rates at different sites on the blank mold is the wear rate of the coating material or solid film lubricant layer for that particular mold. The overall wear rate for the particular solid film lubricant layer, which is reported, is calculated by taking the average of the wear rates of each individual mold that was coated with that material and run in the glass forming machine. The number of blank molds used with a given coating material was always in excess of two and most typically four.
The lifetime of a mold with the solid film lubricant layer is, of course, dependent on the wear rate and also on the thickness of the layer. A highly significant, additional factor, regarding lifetime, is the degree of uniformity of loading of the formable glass into the mold. That is, in order to obtain lifetimes which would be projected from the wear rates it is extremely important that the formable glass be uniformly loaded into the mold so as to preclude any localization of wear in one area. Such localization, of course, gives a misleading lifetime which does not fairly represent the characteristics of the solid film lubricant layer since only a small spot or area may indicate the lifetime has been exceeded but if uniform loading had been obtained a significantly higher lifetime would be realized. The lifetimes reported in the examples are the ranges for the respective molds where glass quality became undesirable.